Transmission apparatus and method using pre-distortion

ABSTRACT

The present invention relates to a transmission apparatus and a corresponding transmission method for transmitting data within a multi-carrier transmission system comprising two or more transmission apparatuses that are configured to transmit the same data. To avoid destructive interferences a transmission apparatus ( 10 ) is proposed comprising a signal input ( 30 ) configured to receive multi-carrier signals (S(k)) carrying data to be transmitted, a distortion unit ( 32 ) configured to distort said multi-carrier signals (S(k)) by use of a distortion function (P(k)) including a phase parameter for differently modulating the phase of said multi-carrier signals (S(k)), wherein said distortion function (P(k)) is different from distortion functions used by other transmission apparatuses, whose coverage areas overlap with the coverage area of the present transmission apparatus, by using a phase parameter that is different from the phase parameter used by said other transmission apparatuses, and a transmission unit ( 34 ) configured to transmit said distorted multi-carrier signals as transmission signal (Tx(k)).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/992,830 filed Aug. 15, 2013, the entire content of which isincorporated herein by reference and is based upon and claims thebenefit of priority from International Application No. PCT/EP11/070554filed Nov. 21, 2011, and pursuant to 35 U.S.C. 119, claims the benefitof priority of European Application No. 10194605.1 filed Dec. 10, 2010.

FIELD OF INVENTION

The present invention relates to a transmission apparatus and acorresponding transmission method for transmitting data within amulti-carrier transmission system comprising two or more transmissionapparatuses that are configured to transmit the same data. Further, thepresent invention relates to a corresponding receiver apparatus andreceiving method for receiving data in such a multi-carrier transmissionsystem and to such a multi-carrier transmission system. Finally, thepresent invention relates to a computer program for implementing saidmethods and a computer readable non-transitory medium storing such acomputer program.

BACKGROUND OF THE INVENTION

Digital terrestrial broadcast systems typically transmit OFDM(Orthogonal Frequency Division Multiplex) signals in a single frequencynetwork (SFN). In certain scenarios, however, signals arriving fromseveral (at least two) transmission apparatuses, interfere destructivelyand the complete signal is canceled (over almost all subcarriers). Thisbehavior was even measured in different field trials, where there arespots in the landscape, which are permanently in deep fade. When thedifferent transmission apparatuses have slightly different frequencies,then these spots move with time, yielding bad reception conditions invarious places.

The application of MIMO (Multiple Input Multiple Output) is also anoption for increased payload bit-rates and robustness in the currentlydeveloped DVB-NGH (Digital Video Broadcasting-Next Generation Handheld)standard. However, the application of MIMO in a broadcasting environmenthas certain drawbacks. Firstly, it is commonly known that MIMO does notoffer high gain for low signal to noise ratios (SNR) if no feedback fromthe receiver to the transmitter is available, which is most likely thecase for DVB-NGH. Secondly, compared to SISO (Single Input SingleOutput), or SIMO (Single Input Multiple Output) (i.e. equivalent to thealready deployed reception diversity), the application of MIMO requiresadditional pilots for channel estimation. Especially for largernetworks, which typically yield many echoes in the corresponding impulseresponses, even SISO transmission requires a high pilot overhead toestimate the highly frequency selective channel. This pilot overhead mayconsume the complete gain offered by MIMO, if the pilot density has tobe doubled. Furthermore, the requirement to estimate additional channelsleads to additional noise in the estimated channel coefficients.

For instance, the efficiency of the MISO (Multiple Input Single Output)scheme for DVB-T2 (which also uses MIMO pilots) has shown a significantadditional degradation due to the channel estimation, which is able toreach up to 0.5 dB additional degradation compared to the SISO case.Hence, it is highly desirable for a multi-carrier transmission system toobtain the MIMO gain in terms of additional diversity, but withouthaving the drawback of the MIMO channel estimation.

SUMMARY OF INVENTION

It is an object of the present invention to provide a transmissionapparatus and a corresponding transmission method that provide increasedrobustness and avoid signal losses if the same data are transmitted bytwo or more transmission apparatuses arranged in the same coverage area.It is a further object of the present invention to provide acorresponding receiver apparatus and receiving method as well as acorresponding transmission system. Finally, it is an object to provide acomputer program for implementing said methods and a computer readablenon-transitory medium.

According to an aspect of the present invention there is provided atransmission apparatus for transmitting data within a multi-carriertransmission system comprising two or more transmission apparatuses thatare configured to transmit the same data, comprising

a signal input configured to receive multi-carrier signals carrying datato be transmitted,

a distortion unit configured to distort said multi-carrier signals byuse of a distortion function including a phase parameter for differentlymodulating the phase of said multi-carrier signals, wherein saiddistortion function is different from distortion functions used by othertransmission apparatuses, whose coverage areas overlap with the coveragearea of the present transmission apparatus, by using a phase parameterthat is different from the phase parameter used by said othertransmission apparatuses, and

a transmission unit configured to transmit said distorted multi-carriersignals as transmission signal.

According to a further aspect of the present invention there is provideda receiver apparatus for receiving data within a multi-carriertransmission system comprising two or more transmission apparatuses thatare configured to transmit the same data, comprising:

a reception input configured to receive a receive signal, said receivesignal substantially corresponding to one or more transmission signalstransmitted by transmission apparatuses within the reception area of thereceiver apparatus, wherein a transmission signal corresponds todistorted multi-carrier signals, said multi-carrier signals beingdistorted by use of a distortion function including a phase parameterfor differently modulating the phase of said multi-carrier signals,wherein said distortion function by a transmission apparatus isdifferent from distortion functions used by other transmissionapparatuses, whose coverage areas overlap with the coverage area of thepresent transmission apparatus, by using a phase parameter that isdifferent from the phase parameter used by said other transmissionapparatuses, wherein the phase parameter is determined by use of a phasefunction, which is identical for all transmission apparatuses within thereception area of the receiver apparatus, and a transmitteridentification sequence, which is unique for each transmission apparatuswithin the reception area of the receiver apparatus,

a signal evaluation unit configured to evaluate said receive signal andretrieve the multi-carrier signals, and

an identification unit configured to identify one or more transmissionapparatuses, from which the one or more transmission signals included insaid receive signal have been transmitted, by identification of saidtransmitter identification sequence from said different phasedistortions of the one or more transmission signals included in saidreceive signal.

According to a further aspect of the present invention there is provideda multi-carrier transmission system comprising two or more of suchtransmission systems and at least one receiver apparatus, in particulara receiver apparatus as explained above.

According to still further aspects a computer program comprising programmeans for causing a computer to carry out the step of distorting of thetransmission method and/or the steps of evaluating and identifying ofthe receiving method according to the present invention, when saidcomputer program is carried out on a computer, as well as a computerreadable non-transitory medium having instructions stored thereon which,when carried out on a computer, cause the computer to perform the stepof distorting of the transmission method and/or the steps of evaluatingand identifying of the receiving method according to the presentinvention are provided.

Preferred embodiments of the invention are defined in the dependentclaims. It shall be understood that the various claimed entities, i.e.apparatuses, methods, transmission system, computer program and computerreadable medium, have similar and/or identical preferred embodiments asthe claimed transmission apparatus and as defined in the dependentclaims.

To circumvent the above described effect of destructive interferences intransmission systems in which several transmission apparatuses transmitthe same data, i.e. that signals are received in such a way that theycancel each other out (mostly determined by the phases of thesubcarriers), the present invention proposes a pre-distortion of thetransmitted signals. The transmission apparatuses, whose coverage areasoverlap, i.e. from which a particular receiver apparatus receivessignals, pre-distort the signal in a different and, preferably, randomor almost random-like way. From the point of view of the transmissionapparatuses, this would correspond to a MISO (Multiple Input SingleOutput) scenario. However, the receiver apparatus treats thesesuperimposed signals as a SISO signal. With these pre-distorted signals,the likelihood of complete signal losses is reduced.

In preferred embodiments special care is taken such that thepre-distortion, which the receiver interprets as being part of thetransmit channel, does not introduce a prolonged channel impulseresponse. Finally, in an embodiment measures are proposed how theproposed method can be used to allow identification of the differenttransmission apparatuses, which could, for instance, be of importancefor field measurements, but also for handover mechanisms in movinghandheld receivers or for efficient monitoring of the multipletransmitters.

Hence, according to the present invention a scheme is proposed which isable to obtain the additional diversity gain, but without having theadditional pilot overhead. The receiver will interpret the detectedsignal as if it is transmitted from one transmission apparatus only(SISO or SIMO). Furthermore, the present invention is suitable to mixthe transmission of MIMO and SISO signals as will be explained below.This is problematic, as transmission apparatuses can not simply beswitched on and off quickly enough if only a single transmissionapparatus is required for SISO operation. In contrast, the transmissionof identical signals by two or more transmission apparatuses would leadto strong additional frequency selectivity, and as simulations haveshown, will drastically reduce the performance of the SISO transmission.

It shall be noted that the present invention relates, for instance, tothe field of Digital Video Broadcasting (DVB) utilizing OrthogonalFrequency Division Multiplexing (OFDM). Further, the present inventioncan generally be applied in other broadcast systems, such as DAB(Digital Audio Broadcasting), DRM (Digital Radio Mondial), MediaFlo,ISDB systems or a future ATSC system, but also in other multi-carriertransmission systems such as communications systems (e.g. an LTE system)in which during handover the base stations (transmitters) handing overan existing communication to a mobile station are simultaneouslytransmitting the same data for a period of time. It should also be notedthat the invention is not limited to the use of OFDM, but can generallybe applied in all multi-carrier transmission systems and theircomponents for the transmission of multi-carrier signals.

It shall further be noted that the transmission of the “same data” shallbe understood as meaning that the same content data is transmitted,which content data have been encoded and/or modulated in the same wayand shall be transmitted generally by use of the same transmissionparameters, e.g. the same bandwidth, except for the herein proposeddistortions avoiding the above described problems.

BRIEF DESCRIPTION OF DRAWINGS

These and other aspects of the present invention will be apparent fromand explained in more detail below with reference to the embodimentsdescribed hereinafter. In the following drawings

FIG. 1 shows a schematic layout of a transmission system according tothe present invention,

FIG. 2 shows a block diagram of a transmission apparatus according tothe present invention,

FIG. 3 shows an example of 15 cos-roll-off functions for use in thedistortion function,

FIG. 4 shows an example of 15 exponential functions for use in thedistortion function,

FIG. 5 shows the absolute squared value over a complete OFDM signal andthe resulting ripple,

FIG. 6 shows an example of an absolute value of a distortion functionfor a transmitter identification function,

FIG. 7 shows combined signals of two transmitters,

FIG. 8 shows a block diagram of a receiver apparatus according to thepresent invention, and

FIG. 9 shows a diagram illustrating an embodiment alternately using aMIMO scheme and a SISO scheme for transmitting data.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates an embodiment of a multi-carriertransmission system 100 according to the present invention. Thetransmission system 100 comprises three transmission apparatuses (Tx)10, 12, 14 and several receiving apparatuses (Rx) 20, 22, 24, 26, 28.Each transmission apparatus 10, 12, 14 has a certain coverage area 11,13, 15 (indicated by dotted or dashed lines) in which a receiverapparatus can receive signals from the respective transmissionapparatus. For instance, the receiving apparatus 22 present in thecoverage area 13 can receive signals only from the transmissionapparatus 12. The coverage areas 11, 13, 15 of the various transmissionapparatuses 10, 12, 14 overlap in certain overlap areas 16, 17, 18, 19so that receiver apparatuses 26, 28 present in such an overlap area 17,19 receive signals from two or more transmission apparatuses 10, 12, 14,e.g. the receiver apparatuses 26 present in the overlap area 17 receivessignals from the transmission apparatuses 10 and 14.

This might lead to the above described problems of destructiveinterferences leading to bad reception quality or even signal losses,particularly if transmission apparatuses whose coverage areas overlap asshown in FIG. 1 transmit the same data (in particular the same contentencoded/modulated in the same way and by used of the same transmissionparameters), e.g. in a broadcast system or in a communications systemduring handover. This shall particularly be avoided according to thepresent invention.

Accordingly, transmitter apparatuses are proposed according to thepresent invention as schematically shown in FIG. 2 for use in thetransmission system as shown in FIG. 1. Such a transmitter apparatus 10(the other transmitter apparatuses 12, 14 have a correspondingconfiguration) comprises a signal input 30 for receiving multi-carriersignals S(k) (e.g. OFDM signals, as will be assumed in the followingexplanation) carrying data to be transmitted. The received OFDM signalsS(k) are provided to a distortion unit 32 for distorting said OFDMsignals S(k) by use of a distortion function P(k) including a phaseparameter for differently modulating the phase of said OFDM signalsS(k). Said distortion function P(k) is different from distortionfunctions used by other transmission apparatuses 12, 14, whose coverageareas 13, 15 overlap with the coverage area 11 of the presenttransmission apparatus 10, by using a phase parameter that is differentfrom the phase parameter used by said other transmission apparatuses 12,14. The distorted OFDM signals S′(k) are provided to a transmission unit34 (including a transmission antenna) for transmitting said distortedOFDM signals S′(k) as transmission signal Tx(k).

One idea behind the present invention is the avoidance of any regularstructure of the resulting fades of two or more transmission apparatuses(also simply called “transmitters” in the following) having overlappingcoverage areas as shown in FIG. 1, in particular when using a SISO mode(or in MIMO or MISO mode, where the same problem could appear, but witha smaller negative impact) in which all transmitters send the samesignal (comparable to an SFN system). As the signals of bothtransmitters have to be added in amplitude and phase, the signals maycancel each other in specific parts of the network. Due to highcorrelation of the signals of the two or more transmitters at a receiverpresent in an overlap area, the signals may even cancel each other overthe complete signal bandwidth. However, the effect that the differenttransmission paths cancel each other completely can be avoided, if thephases of at least all but one transmitters (e.g. one transmitter incase of two transmitters having overlapping coverage areas) are modifiedover the subcarriers. Then, the case that all data is lost does notoccur.

The present invention in an embodiment introduces a random orrandom-like structure of the phases between the two or moretransmitters. Hence, the two or more transmitters still transmit thesame data, but the phases of the data are modified differently, e.g. ina random or random-like way. This does not remove the destructiveinterference within the network, but it spreads this interferenceequally over the complete signal bandwidth and the complete receptionarea. However, a simple random-like structure of the phases between thetwo or more transmitters is not desired, because it should still bepossible using the normal (e.g. SISO) channel estimation. This will beexplained in the following.

Let S(k) be an original OFDM signal (generally, a multi-carrier signal)in its frequency domain representation, where k is the OFDM subcarrierindex. Now, the original signal S(k) is linearly distorted by means of adistortion function P(k), which finally leads to the transmitted signalTx(k):Tx(k)=S(k)·P(k,tx).  (1)

The distortion function P(k, tx) also depends on the OFDM subcarrier k.Furthermore, the signal P(k, tx) shall be different (preferably unique)for each of the two or more transmitters (indicated by index tx; in thefollowing also the notations P_(tx)(k) or simply P(k) are used instead)having overlapping coverage areas within the network to ensure highestdecorrelation between them. Additionally, the amplitudes of thedistorted OFDM subcarriers shall preferably remain constant, while thephases shall be changed to ensure decorrelation. Hence, the distortionfunction can be expressed by means of a complex phase rotation, i.e.P(k)=e ^(j2πφ(k,tx))  (2),where the phase Φ depends on the OFDM subcarrier k and the transmitter.

Preferably, the application of the linear pre-distortion by thedistortion function P(k) shall be as invisible as possible to thereceiver apparatus (also simply called “receiver” in the following).Assuming a system with one transmitter, in case of an ideal OFDM system,the linear distortions caused by the channel within the frequency domaincan be modeled by means of a complex multiplication of the transmittedsignal Tx(k) with the complex channel transfer function H(k) on thecorresponding OFDM subcarrier k. This leads toR(k)=H(k)·Tx(k)=H(k)·(S(k)·P(k)),  (3)where R(k) is the received value of the OFDM subcarrier k. By means ofan inverse Fourier transform, equation (3) can be expressed within thetime domain asr(t)=h(t)*(s(t)*p(t))=[h(t)*p(t)]*s(t)=h _(eq)(t)*s(t).  (4)The small letters are the time domain representation of thecorresponding frequency domain representation, while * denotes theconvolution. As already mentioned, the pre-distortion shall be fullytransparent to the receiver. Hence, the receiver sees the equivalentchannel impulse response h_(eq)(t) and its frequency domainrepresentation H_(eq)(k), which is the Fourier transform of h_(eq)(t).Within h_(eq)(t), h(t) is the actual impulse response and p(t) is theartificial impulse response caused by the linear pre-distortion. As alonger impulse response h_(eq)(t) requires more pilot signals for thesampling, and thus, the equalization of the channel, the term p(t)should be as short as possible. Consequently, the phase term in equation(2) is preferably chosen such that the width of the Fourier transform ofthis term remains a narrow as possible, as this is exactly theartificial broadening of the equivalent channel impulse responseh_(eq)(t) seen by the receiver. A broad artificial impulse response p(t)requires a higher pilot density for the sampling of the resultingchannel transfer function. However, it is preferred to avoid this and tokeep this artificial impulse response as narrow as possible.

Equation (2) can also be seen as a frequency modulation of the originalOFDM data in the frequency domain. The “spectrum” of this frequencymodulated signal is exactly the resulting artificial impulse responsep(t) seen by the receiver. A simple solution to this problem, i.e. theproblem of avoiding a higher pilot density and having a too broadartificial impulse response, is the application of cyclic delaydiversity. The signal of one of the transmitters is delayed. Inequations (1) and (2) this leads toTx(k)=S(k)·P(k) with P(k)=e ^(j2πφ(k)) =e ^(−j2πΔk),  (5)where Δ is the (normalized) delay of the signal. Within the time domainrepresentation, this would lead toTx(t)=s(t)*p(t)=s(t−Δ)  (6)for the signal of the first transmitter. The signal of the secondtransmitter (in case there are only two transmitter having overlappingcoverage area) is not modified. This solution works for the co-locatedtransmission, in which transmitters (more precisely, the transmissionantennas) are located very closely to each other, of the SISO signalfrom the two MIMO antennas. Furthermore, the resulting length of theartificial impulse response is just the delay Δ. However, there couldstill exist some regions in the network which may suffer from fades overthe complete bandwidth. This is especially the case for Single FrequencyNetworks (SFN). A delay of one transmitter only moves the positions ofthe fades within the network area, but it does not completely avoid it.

Therefore, equations (1) and (2) can also be used with a differentmodulation of the phase. Although equation (5) is a modulation, whichresults in a finite length of artificial impulse response, and mostother solutions will lead to an infinite length of the resulting impulseresponse, a finite length is achievable, if slight variations of theamplitude P(k) are accepted. For this purpose, different approaches maybe used as will be explained in the following by use of differentexamples.

In order to limit the broadening of the impulse response, multipleRaised Cosine Functions (also called cos-roll-off function) are used inan embodiment to generate the distortion function (also calledpre-coding signal) P(k). Such a cos-roll-off function still has aninfinite broadening of the impulse response, but the slopes fall quitesteeply if the roll-off-factor is chosen quite high. Therefore, thisfilter practically leads to a finite length of the artificial impulseresponse p(t), which is the inverse Fourier transform of the frequencydomain representation of the filter, i.e.

$\begin{matrix}{{W_{RC}(k)} = \left\{ {\begin{matrix}1 & {{{if}\mspace{14mu}{k}} \leq \frac{1 - \alpha}{2T}} \\{\cos^{2}\left( {\frac{\pi\; T}{2\alpha}\left( {{k} - \frac{1 - \alpha}{2T}} \right)} \right)} & {{{if}\mspace{14mu}\frac{1 - \alpha}{2T}} < {k} \leq \frac{1 + \alpha}{2T}} \\0 & {else}\end{matrix},} \right.} & (7)\end{matrix}$where W_(RC)(k) is also generally called a window function herein, T isa (designable) time constant and α a design constant (the so-calledroll-off factor) in the range from 0 to 1. If α=1 is selected the slopesof the time domain signal fall off most steeply (among this set offunctions), while for α=0 the slopes fall off least steeply. However,the window function W_(RC)(k) is an ideal rectangular filter for α=0,but for α=1 there are slopes (in frequency domain) of W_(RC)(k), whichwill affect the edges of the multi-carrier signal. In practice, a ispreferably selected in the middle range between 0 and 1.

The sum of multiple correctly aligned cos-roll-off functions in thefrequency domain then gives the flat spectrum. However, in principleeach cos-roll-off function can be modulated separately, while themaximum broadening of the impulse response is given by the time domaindescription of the cos-roll-off function. Hence, the distortion functionP(k) can be described in the frequency domain as

$\begin{matrix}{{{P(k)} = {\sum\limits_{l = 1}^{L - 1}\left\lbrack {e^{{j2\pi\Psi}{(l)}} \cdot {W_{RC}\left( {k - {l \cdot \frac{N}{L}}} \right)}} \right\rbrack}},} & (8)\end{matrix}$where W_(RC)(k) is the frequency domain description of the cos-roll-offfunction, N is the total number of OFDM subcarriers (with index k) and Lthe number of cos-roll-off functions (generally, the number of windowfunctions), into which the spectrum is divided (the index is given by1). Furthermore, the phase term Ψ(l) (also called phase parameter) isconstant for each of the roll-off function, but may vary between thedifferent roll-off-functions (indicated by Ψ(l) in equation (8), wherethis phase is constant for all subcarriers k in the windowing range ofW_(RC)(k−1*N/L)). However, multi-carrier systems like OFDM systemsnormally do not utilize the maximum number of available carriers N, butthey leave some carriers at the edges of the spectrum unmodulated inorder to avoid disturbance to neighboring channels. Consequently, thesignal is preferably divided into L segments, but only L−1 segments areused.

The actual length of the artificial impulse response only depends on theterm W_(RC)(k), which is caused by the linearity of the sums in equation(8). As the phase term of each sum is constant, and the delay term(1*N/L) does not cause any change to the absolute value of the impulseresponse, the maximum length of the artificial impulse response (i.e.the sum) cannot be longer than the maximum length of each summand.

In an example related to the DVB-T2 8K OFDM mode the variables N and Lare set to N=8192 and L=16. Furthermore, the timing constant T can beset such that the different cos-roll-off functions are well aligned andadd to 1, i.e.T=L/N=16/8192.  (9)

The factor α can be chosen freely, depending on the maximum allowedlength of the resulting artificial impulse response. FIG. 3 shows the 15resulting cos-roll-off functions and their sum, which is equal to 1within the range of the actually modulated OFDM subcarriers.

Another example for a window function to obtain the desiredcharacteristics is the exponential function. It can replace the termW_(RC) in equation (8) byW _(exp)(k)=e ^(−π(kT)) ²   (10)where the time constant T is similar to equation (9). FIG. 4 depicts theresults.

In principle, each function that is well localized in time and frequency(i.e. a function with a compromise of having a short impulse responseand a steep slope in frequency domain) can be used as window function Win the definition of the distortion function P

$\begin{matrix}{{P(k)} = {\sum\limits_{l = 1}^{L - 1}\left\lbrack {e^{{j2\pi\Psi}{(l)}} \cdot {{W\left( {k - {l\frac{N}{L}}} \right\rbrack}.}} \right.}} & (11)\end{matrix}$Further examples are the well-known Hamming window, Hann window, or asinc²-function etc. In other embodiments, a spectrum of a Nyquistimpulse is used as said window function, preferably a spectrum of such aNyquist impulse whose time domain representation drops as fast aspossible to zero.

Hence, in a preferred embodiment the distortion unit 32 uses adistortion function P(k) comprising a sum of two or more windowfunctions W, in particular identical window functions, each windowfunction covering a set of a plurality of subcarriers in the frequencydomain. Said sets are overlapping or adjacent to each other, inparticular such that the sum of all window functions is substantiallyconstant over the complete bandwidth covered by the subcarriers in thefrequency domain. Each window function W of said sum is multiplied witha phase function e^(j2πΨ(l)) including said phase parameter Ψ(l),wherein said phase parameter is preferably constant within a phasefunction (in general, the phase parameter is linear increasing ordecreasing over the frequency). Said phase parameter Ψ(l) is preferablydifferent within the different phase functions.

For ensuring decorrelation between different transmitters, the phaseparameter Ψ(l) of each summand in equation (8) is preferably modulateduniquely for each transmitter within a network. This ensuresuncorrelated fading, and additionally can be used to identify eachtransmitter within the network. Therefore, the phase is modulated in anembodiment by the transmitter identification sequence c as follows:Ψ(0)=c(l)Ψ(l)=Ψ(l−1)+c(l)/8 with cε{−1,0,1}.  (12)

Equation (12) is a differential modulation, which changes the phases inequation (8) from summand to summand. Preferably, the phase rotationbetween two consecutive summands is limited, e.g. to π/4 in case ofequation (12). However, other limitation values (angles) are alsopossible. FIG. 5 shows the reason for the cos-roll-off function. Theoverlapping parts of two cos-roll-off functions have to be added in thecomplex domain. In case of phase changes, the resulting absoluteamplitude is less than 1. However, the ripples in this example are only0.7 dB deep and will most probably not have any effect. Furthermore, themean power of P(k) can be normalized to 1. It is a general problem thatthese ripples occur. They can only be avoided in case of cyclic delaydiversity (see equation (5)) or if the broadening of the impulseresponse is infinite, which does not allow sampling the channel transferfunction using the pilots. It shall be noted here that a selection ofα=1 (see equation (7)) would lead to a broader signal drop in the signalshown in FIG. 5.

Next, an example shall be explained for a transmission system having twotransmitters. For obtaining highest decorrelation, each transmitter isprovided with a unique identification sequence c. As an example, thefirst transmitter (indicated by the index 1 in the following) gets thetransmitter identification sequence c₁=(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0)(i.e. 15 zeros). This actually means that the signal is not modified andthe phase parameter applied by the first transmitter isΨ₁=(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0).The second transmitter is provided with the transmitter identificationsequence c₂=(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) (i.e. 15ones). Hence, the phase changes from summand to summand and the phaseparameter applied by the first transmitter isΨ₂=(1/8,1/4,3/8,1/2,5/8,3/4,7/8,1,9/8,5/4,11/8,3/2,13/8,7/4,15/8),i.e. the phase rotates by π/4 from summand to summand in equation (8).FIG. 6 shows the absolute value of the obtained distortion function P(k)for the second transmitter. Due to the effect already presented in FIG.5, the transitions from the different cos-roll-off functions show slightripples.

The received signals from both transmitters as (see equation (3)) can bewritten asR(k)=H ₁(k)P ₁(k)·S ₁(k)+H ₂(k)P ₂(k)·S ₂(k),  (13)where P_(tx)(k) is the linear pre-distortion, and H_(tx)(k) is thechannel transfer function from the different transmitters (indicated byindex tx), and where any additive noise part is neglected forsimplicity. As both transmitters transmit the same signal, i.e.S₁(k)=S₂(k)=S(k), equation (13) can be simplified intoR(k)=[H ₁(k)P ₁(k)+H ₂(k)P ₂(k)]·S(k).  (14)

The resulting signal for S(k)=1 and H₁(k)=H₂(k)=1 is shown in FIG. 7showing combined signals of both transmitters, the signals fade onspecific subcarriers (e.g. around 2000, 6000), but it is almostimpossible that they fade on all subcarriers. The signals fade atspecific OFDM subcarriers (or frequencies). However, due to linearpre-distortion it is extremely unlikely that both signals cancel eachother completely, which would require that H₁(k)P₁(k)+H₂(k)P₂(k)=0 onall subcarriers k.

An important aspect of the transmitter identification sequence c is thepossibility to identify a specific transmitter, e.g. within a singlefrequency network. A receiver can identify the differential phasechanges between two different summands in equation (8). If the receiverhas knowledge about the different identification sequences c in anetwork, the receiver can identify each transmitter by means of thissequence. This can, for instance, be used by the network operator tomonitor the proper operation of each transmitter. Furthermore, it canalso be used by a receiver to estimate its current position in thenetwork.

A possible implementation of a receiver apparatus, as an example thereceiver apparatus 26 shown in FIG. 1 for transmitter identificationshall be explained below with reference to FIG. 8. The receiverapparatus 26 comprises a reception input 40 configured to receive areceive signal Rx(k). Said receive signal Rx(k) substantiallycorresponds to one or more transmission signals Tx(k) transmitted bytransmission apparatuses (10, 14 within the reception area of thereceiver apparatus 26. A transmission signal (Tx) corresponds, asexplained above, to distorted multi-carrier signals, said multi-carriersignals S(k) being distorted by use of a distortion function P(k)including a phase parameter for differently modulating the phase of saidmulti-carrier signals S(k), wherein said distortion function P(k) usedby a transmission apparatus is different from distortion functions usedby other transmission apparatuses, whose coverage areas overlap with thecoverage area of the present transmission apparatus, by using a phaseparameter that is different from the phase parameter used by said othertransmission apparatuses, wherein the phase parameter is determined byuse of a phase function, which is identical for all transmissionapparatuses within the reception area of the receiver apparatus, and atransmitter identification sequence, which is unique for eachtransmission apparatus within the reception area of the receiverapparatus 26.

The receiver apparatus 26 further comprises a signal evaluation unit 42for evaluating said receive signal Rx(k) and retrieving themulti-carrier signals S(k). Further, the receiver apparatus 26 furthercomprises an identification unit 44 configured to identify one or moretransmission apparatuses Tr, from which the one or more transmissionsignals included in said receive signal Rx(k) have been transmitted, byidentification of said transmitter identification sequence c from saiddifferent phase distortions of the one or more transmission signalsincluded in said receive signal Rx(k). This identification will beexplained for an embodiment in more detail.

It shall be assumed that the receiver is able to completely decode thereceived signal. Hence, the receiver is able to locally reconstruct thetransmitted signal S(k). Additionally, the used transmitteridentification sequences c within the network are also known to thereceiver. Consequently, the receiver is also able to locally reconstructthe distortion signals P_(x)(k). However, the channel transfer functionsH_(x)(k) are unknown, because it is difficult for the receiver toseparate the summed signal of the two transmitters.

A correlation process is used to identify the transmitters. Therefore,the time domain representation of the OFDM signals (i.e. the inverseFourier transform), is needed, which are described by small letters.Thus, equation (14) can be reformulated asr(t)=[h ₁(t)*p ₁(t)+h ₂(t)*p ₂(t)]*s(t)  (15)where the * denotes a convolution operation.

Furthermore, the receiver generates the following signal,e _(x)(t)=p _(x)(t)*s(t),  (16)which is the time domain representation of the distortion functionP_(x)(k) convoluted with the transmitted signal S(k).

Furthermore, it shall be assumed that the sequences c have been chosensuch that the time domain representation of the transmitter sequencep_(x)(t) are almost orthogonal, i.e., they possess vanishingcross-correlation functions. Then, equations (15) and (16) are combinedusing a correlation process:corr=r(t)

e_(x)(t)=r(−t)=*e _(x)(t).  (17)

The symbol

denotes the correlation, which is equivalent to the convolutionoperation, if the time axis of the first term is mirrored. Furthercalculations of equation (17) lead to

$\begin{matrix}\begin{matrix}{{corr} = {{r\left( {- t} \right)}*{e_{x}(t)}}} \\{= {\left( {{{h_{1}\left( {- t} \right)}*{p_{1}\left( {- t} \right)}} + {{h_{2}\left( {- t} \right)}*{p_{2}\left( {- t} \right)}}} \right)*{s\left( {- t} \right)}*{p_{x}(t)}*{s(t)}}} \\{{= {{\varphi_{ss}(t)}*\left( {{{h_{1}\left( {- t} \right)}*{p_{1}\left( {- t} \right)}} + {{h_{2}\left( {- t} \right)}*{p_{2}\left( {- t} \right)}}} \right)*{p_{x}(t)}}},}\end{matrix} & (18)\end{matrix}$where ω_(ss)(t) denotes the auto-correlation of s(t).

Now x=1 can be set in equation (18) in order to identify the firsttransmitter:

$\begin{matrix}\begin{matrix}{{corr} = {{\varphi_{ss}(t)}*\left( {{{h_{1}\left( {- t} \right)}*{p_{1}\left( {- t} \right)}} + {{h_{2}\left( {- t} \right)}*{p_{2}\left( {- t} \right)}}} \right)*{p_{1}(t)}}} \\{= {{\varphi_{ss}(t)}*{\left( {{{h_{1}\left( {- t} \right)}*{p_{1}\left( {- t} \right)}*{p_{1}(t)}} + {{h_{2}\left( {- t} \right)}*{p_{2}\left( {- t} \right)}*{p_{1}(t)}}} \right).}}}\end{matrix} & (19)\end{matrix}$

As it can be assumed that the p_(x)(t) are orthogonal, equation (19)simplifies to

$\begin{matrix}\begin{matrix}{{corr} = {{\varphi_{ss}(t)}*\left( {{{h_{1}\left( {- t} \right)}*{p_{1}\left( {- t} \right)}*{p_{1}(t)}} + {{h_{2}\left( {- t} \right)}*{p_{2}\left( {- t} \right)}*{p_{1}(t)}}} \right)}} \\{= {{\varphi_{ss}(t)}*{\left( {{h_{1}\left( {- t} \right)}*{p_{1}\left( {- t} \right)}*{p_{1}(t)}} \right).}}}\end{matrix} & (20)\end{matrix}$

Due to the orthogonality, the term of the second transmitter almostdisappeared. Hence, the value of con only depends on the transmitter 1.If the absolute value of corr is above a certain threshold, thetransmitter has been detected in the network.

In an embodiment the receiver apparatus 26 further comprises anestimation unit 46 configured to estimate the current position of thereceiver apparatus in the transmission system by use of the identifiedtransmission apparatuses.

The present invention may also be used in other scenarios. For instance,in a scenario transmission devices are used, each having two or moretransmission apparatuses (which can, for instance, be two or moretransmission antennas) for transmitting data in different MIMO modes.Here, the term MIMO mode shall not be construed as being limited toselecting a MIMO (Multiple Input Multiple Output) antenna configuration,using at least two antennas for transmission in the transmitter and atleast two antennas for reception in the receiver. In contrast, othermodes and, thus, other antenna configurations shall also be availablefor selection, and the term MIMO mode selection shall thus be understoodbroadly in this broad sense. In particular, MIMO mode shall beunderstood as one of a SISO (Single Input Single Output) scheme, MISO(Multiple Input Single Output) scheme, SIMO (Single Input MultipleOutput) scheme, or MIMO scheme, which represent the most common schemes,i.e. the MIMO mode available for selection can be MIMO, MISO or SISOscheme (often also called “mode” or “antenna configuration”) in thisembodiment.

For instance, in an embodiment as shown in FIG. 9 a first transmissionapparatus (antenna) is adapted for transmission of data blocks mappedonto data frames in any MIMO mode and wherein the further transmissionapparatuses (antennas) are adapted for transmission of data blocksmapped onto data frames in the MISO scheme or MIMO scheme, wherein theone or more further apparatuses (antennas) are adapted for alsotransmitting data during times where the first transmission apparatus(antenna) is transmitting data blocks mapped onto data frames in theSISO scheme, and wherein said further transmission apparatuses(antennas) are preferably adapted for each substantially using the sametransmission power as the first transmission apparatus (antenna).Generally, all transmission apparatuses (antennas) split up the totalavailable transmission power.

This embodiment can generally be used in any kind of transmissionsystem, including broadcast systems, using at least two transmissionapparatuses (antennas) in which different MIMO modes are alternatelyused from time to time, i.e. where it is needed to quickly switch theone or more further transmission apparatuses (antennas) on and off. Suchquick switching operations are thus avoided in such an embodiment of thepresent invention.

Preferably, said one or more further apparatuses (antennas) are adaptedfor transmitting, during times when the first transmission apparatuses(antennas) is transmitting data blocks mapped onto data frames in theSISO scheme, the same data as the first apparatus (antenna). Thiscontributes to avoid undesired power variation among said one or morefurther apparatuses (antennas) and avoids the above described problemsof destructive interferences leading to local signal drops or totalsignal losses.

The invention has been illustrated and described in detail in thedrawings and foregoing description, but such illustration anddescription are to be considered illustrative or exemplary and notrestrictive. The invention is not limited to the disclosed embodiments.Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

A computer program may be stored/distributed on a suitablenon-transitory medium, such as an optical storage medium or asolid-state medium supplied together with or as part of other hardware,but may also be distributed in other forms, such as via the Internet orother wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limitingthe scope.

The invention claimed is:
 1. A transmission apparatus for transmittingdata within a multi-carrier transmission system including two or moretransmission apparatuses that are configured to transmit same data, thesame data being encoded and/or modulated a same way and beingtransmitted using same transmission parameters, the transmissionapparatus comprising: a circuitry configured to: receive subcarriers ofmulti-carrier signals carrying data to be transmitted, distort, usingfilters, said multi-carrier signals based on a distortion function, saiddistortion function having an effect of introducing a phase distortionto the subcarriers by modulating a phase of the subcarriers of themulti-carrier signals, and said distortion function being different fromdistortion functions used by other transmission apparatuses, whosecoverage areas overlap with a coverage area of the transmissionapparatus, such that corresponding carriers that transmit the same datafrom the other transmission apparatuses are filtered differently by theother transmission apparatuses, and transmit said distortedmulti-carrier signals as a transmission signal, wherein application ofsaid distortion function to said multi-carrier signals is invisible to areceiver that receives said distorted multi-carrier signals.
 2. Thetransmission apparatus of claim 1, wherein each of the two or moretransmission apparatuses is configured to transmit identificationinformation.
 3. The transmission apparatus of claim 1, wherein each ofthe two or more transmission apparatuses includes an antenna.
 4. Thetransmission apparatus of claim 1, wherein the filters are frequencydomain filters.
 5. The transmission apparatus of claim 1, furthercomprising Inverse Fourier Transform circuitry.
 6. The transmissionapparatus of claim 1, wherein the circuitry is configured to operate ina multicarrier Multiple Input Single Output (MISO) transmission system.7. The transmission apparatus of claim 1, wherein the circuitry isconfigured to transmit data arranged for receipt by a Multiple InputSingle Output (MISO) receiver.
 8. The transmission apparatus of claim 1,wherein the distortion function includes a phase parameter.
 9. Thetransmission apparatus of claim 1, wherein the distortion functionincludes a sum of two or more window functions, and wherein each windowfunction covers a set of the subcarriers in a frequency domain.
 10. Thetransmission apparatus of claim 9, wherein said two or more windowfunctions are configured to limit a length of an artificial impulseresponse caused by said distortion function.
 11. The transmissionapparatus of claim 9, wherein the sum of said two or more windowfunctions is constant over a complete bandwidth covered by thesubcarriers in the frequency domain.
 12. The transmission apparatus ofclaim 1, wherein sets of the subcarriers overlap with or are adjacent toeach other.
 13. A transmission method for transmitting data within amulti-carrier transmission system including two or more transmissionapparatuses that are configured to transmit same data, the same databeing encoded and/or modulated a same way and being transmitted usingsame transmission parameters, the transmission method comprising:receiving subcarriers of multi-carrier signals carrying data to betransmitted; distorting, using filters, said multi-carrier signals basedon a distortion function, said distortion function having an effect ofintroducing a phase distortion to the subcarriers by modulating a phaseof the subcarriers of the multi-carrier signals, and said distortionfunction being different from distortion functions used by othertransmission apparatuses, whose coverage areas overlap with a coveragearea of a transmission apparatus of the multi-carrier transmissionsystem, such that corresponding carriers that transmit the same datafrom the other transmission apparatuses are filtered differently by theother transmission apparatuses; and transmitting said distortedmulti-carrier signals as a transmission signal, wherein application ofsaid distortion function to said multi-carrier signals is invisible to areceiver that receives said distorted multi-carrier signals.
 14. Thetransmission method of claim 13, wherein the distorting is performed byfiltering in the frequency domain.
 15. A system comprising: atransmission apparatus including circuitry configured to: receivesubcarriers of multi-carrier signals carrying data to be transmitted,distort, using filters, said multi-carrier signals based on a distortionfunction, said distortion function having an effect of introducing aphase distortion to the subcarriers by modulating a phase of thesubcarriers of the multi-carrier signals, and said distortion functionbeing different from distortion functions used by other transmissionapparatuses, whose coverage areas overlap with a coverage area of thetransmission apparatus, such that corresponding carriers that transmitsame data from the other transmission apparatuses are filtereddifferently by the other transmission apparatuses, and transmit saiddistorted multi-carrier signals as a transmission signal; and areceiving apparatus including circuitry configured to: receive saiddistorted multi-carrier signals, wherein application of said distortionfunction to said multi-carrier signals by the transmitting apparatus isinvisible to the receiving apparatus.
 16. The system of claim 15,wherein at the receiving apparatus, the application of said distortionfunction to the multi-carrier signals is interpreted as being part of atransmit channel over which the transmission signal is communicated.